Enclosure Services Interface
# Enclosure Services Interface

The Enclosure Services Interface (ESI) is a computer protocol used in SCSI enclosures. This is part of a chain of connections that allows a host computer to communicate with the enclosure to access its power, cooling, and other non-data characteristics. This overall approach is called SCSI attached enclosure services:

The host computer communicates with the disks in the enclosure via a Serial SCSI interface (which may be either FC-AL or SAS). One of the disk devices located in the enclosure is set up to allow SCSI Enclosure Services (SES) communication through a LUN. The disk-drive then communicates with the SES processor in the enclosure via ESI. The data sent over the ESI interface is simply the contents of a SCSI command and the response to that command.

In fault-tolerant enclosures, more than one disk-drive slot has ESI enabled to allow SES communications to continue even after the failure of any of the disk-drives.

## ESI electrical interface

The ESI interface was designed to make use of the seven existing "SEL_n" address signals which are used at power-on time for establishing the address (ALPA) of a disk-drive. An extra eighth signal called "-PARALLEL ESI" is used to switch the function of the SEL_n signals.

| Signal name | Function |
| - | - |
| SEL_0/D0 | Data bus bit 0 |
| SEL_1/D1 | Data bus bit 1 |
| SEL_2/D2 | Data bus bit 2 |
| SEL_3/D3 | Data bus bit 3 |
| SEL_4/-ENCL_ACK | The enclosure clocks this to acknowledge a read or write data transfer |
| SEL_5/-DSK_RD | The disk-drive clocks this to send a NIBL of data to the enclosure |
| SEL_6/-DSK_WR | The disk-drive clocks this to receive a NIBL of data from the enclosure |

## ESI command sequence

A SCSI Send Diagnostic command or Receive Diagnostic Results command is sent from the host computer to the disk-drive to initiate an SES transfer. The Disk-drive then asserts "-PARALLEL ESI" to begin this sequence of ESI bus phases:

| | Phase | Function |
| - | - | - |
| | Discovery phase | Disk-drive tests that the enclosure is SFF-8067 compliant |
| | Command phase | Disk-drive sends the SCSI CDB to the enclosure (similar to the write phase) |
| Either | Read phase | Disk-drive sends diagnostic page data to the enclosure |
| or | Write phase | Disk-drive receives diagnostic page data from the enclosure |

Finally, the disk-drive deasserts "-PARALLEL ESI".

The above sequence is just a simple implementation of a 4-bit wide parallel interface which is used to execute a SCSI transaction. If the CDB is for a Send Diagnostic command then the data is sent to a SCSI diagnostic page in the enclosure. If the CDB is for a SCSI Receive Diagnostic Results command then the data is received from a SCSI diagnostic page. No other CDB types are allowed.

## Alternatives to ESI

There are two common alternatives ESI:

* Standalone Enclosure Services uses a direct connection which does not require ESI
* SSA enclosures use an interface called DSI which is similar to ESI

## Specifications

The definition of the ESI protocols is owned by an ANSI committee and defined in their specifications ANSI SFF-8067 and ANSI SFF-8045.

